Polyolefin fiber having improved initial elongation and process for preparation thereof

ABSTRACT

A polyolefin fiber having an improved initial elongation, comprises a strongly drawn body of a composition comprising ultra-high-molecular-weight polyethylene and an ultra-high-molecular-weight copolymer of ethylene with an olefin having at least 3 carbon atoms at such a ratio that the content of the olefin having at least 3 carbon atoms in the entire composition is such that the number of side chains per 1000 carbon atoms in the composition is 0.2 to 5.0 on the average, and having an intrinsic viscosity [ eta ] of at least 5 d l /g as the entire composition, wherein the strongly drawn body has at least two crystal melting endothermic peaks, close to each other, in the region of temperatures higher by at least 15 DEG C than the inherent crystal melting temperature (Tm) of the composition determined as the main melting endothermic peak at the second temperature elevation when measured in the restrained state by a differential scanning calorimeter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-strength polyolefin fiber havingan improved initial elongation and a process for the preparationthereof.

2. Description of the Related Art

It is known that a molecularly oriented shaped body having high elasticmodulus and high tensile strength is obtained by shapingultra-high-molecular-weight polyethylene into a fiber, a tape or thelike and drawing the shaped body. For example, Japanese PatentApplication Laid-Open Specification No. 15408/81 discloses a process inwhich a dilute solution of ultra-high-molecular-weight polyethylene isspun and the obtained filament is drawn. Futhermore, Japanese PatentApplication Laid-Open Specification No. 130313/84 discloses a process inwhich ultra-high-molecular-weight polyethylene is melt-kneaded with awax and the kneaded mixture is extruded, cooled, solidified and thendrawn. Moreover, Japanese Patent Application Laid-Open Specification No.187614/84 discloses a process in which a melt-kneaded mixture asmentioned above is extruded, drafted, cooled, solidified and then drawn.

If ultra-high-molecular-weight polyethylene is shaped into a fiber andthe fiber is strongly drawn, the elastic modulus and tensile strengthare increased with increase of the draw ratio. This drawn fiber has goodmechanical properties such as high elastic modulus and high tensilestrength and is excellent in light weight characteristic, waterresistance and weatherabililty. However, this drawn fiber is stillinsufficient and defective in that the initial elongation is large andthe creep resistance is poor.

The initial elongation is a phenomenon which is peculiarly and commonlyobserved in organic fibers, and this phenomenon is observed even inrigid high polymers such as a Kevlar fiber (wholly aromatic polyamidefiber). Especially in the above-mentioned polyethylene fiber having highelastic modulus and high strength, the initial elongation is so large asabout 1% at normal temperature, and a high elastic modulus cannot besufficiently utilized in the field of, for example, a composite materialor the like. More specifically, influences by this large initialelongation are serious in fiber-reinforced resin composite materials,tension members (optical fiber cords) and the like.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide apolyolefin fiber which is highly improved in the initial elongation andcreep resistance and has high strength and elastic modulus and a processfor the preparation of this polyolefin fiber.

More specifically, in accordance with one aspect of the presentinvention, there is provided a polyolefin fiber having an improvedintitial elongation, which comprises a strongly drawn body of acomposition comprising ultra-high-molecular-weight polyethylene and anultr-high-molecular-weight copolymer of ethylene with an olefin havingat least 3 carbon atoms at such a ratio that the content of the olefinhaving at least 3 carbon atoms in the entire composition is such thatthe number of side chains per 1000 carbon atoms in the composition is0.2 to 5.0 on the average, and having an intrinsic viscosity [η] of atleast 5 dl/g as the entire composition, wherein the strongly drawn bodyhas at least two crystal melting endothermic peaks, close to each other,in the region of temperature higher by at least 15° C. than the inherentcrystal melting temperature (Tm) of the composition determined as themain melting endothermic peak at the second temperature elevation whenmeasured in the restrained state by a differential scanning calorimeter.

In accordance with another aspect of the present invention, there isprovided a process for the preparation of a polyolefin fiber having animproved initial elongation, which comprises melt-kneading a compositioncomprising ultra-high-molecular-weight polyethylene having a intrinsicviscosity [η] of at least 5 dl/g, and an ultra-high-molecular-weightethylene/α-olefin copolymer having an intrinsic viscosity [η] at least 5dl/g, said α-olefin having at least 3 carbon atoms, and having such acontent of the α-olefin having at least 3 carbon atoms that the numberof side chains of the α-olefin per 1000 carbon atoms in the copolymer is0.5 to 10 on the average, at a weight ratio of from 10/90 to 90/10, inthe presence of a diluent, spinning the kneaded mixture and drawing theobtained fiber at a draw ratio of at least 10.

When a load corresponding to 30% of the breaking load at roomtemperature is applied to the polyolefin fiber of the present inventionat a sample length of 1 cm and an ambient temperature of 70° C., theinitial elongation after 60 seconds from the point of the initiation ofthe load is lower than 5% and the average creep speed during the periodof from the point of 90 seconds from the initiation of application ofthe load to the point of 180 seconds from the initiation of applicationof the load is lower than 1×10⁻⁴ sec⁻¹. These characteristics of thefiber of the present invention are quite surprising.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the creep characteristics of anultra-high-molecular-weight polyethylene fiber (4), andultra-high-molecular-weight ethylene/butene-1 copolymer fiber (5) andfibers (1) through (3) of compsitions of both the polymers.

FIGS. 2, 3, 4, 5 and 6 are differential thermal curves of the foregoingsamples (1) through (5).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is based on the finding that a compositioncomprising ultra-high-molecular-weight polyethylene and anultra-high-molecular-weight copolymer of the ethylene with an α-olefinhaving at least 3 carobn atoms (hereinafter referred to as"ultra-high-molecular-weight ethylene/α-olefin copolymer") at a certainblend ratio is excellent in the spinnability and drawability and can beeasily shaped in a strongly drawn shaped body, and this drawn shapedbody has very high elastic modulus and strength and also has excellentcreep resistance and in this drawn shaped body, the initial elongationis controlled to a very low level.

Ultra-high-molecular-weight polyethylene can be drawn at a high drawratio and the fiber obtained at a high draw ratio shows high strengthand high elastic modulus, but the drawn fiber is defective in that thecreep resistance is poor. On the other hand, a fiber of anultra-high-molecular-weight ethylene/α-olefin copolymer has an excellentcreep resistance, but the drawability is not sufficient and a yarnhaving high strength and high elastic modulus can hardly be obtained. Ahighly drawn fiber comprising ultra-high-molecular-weight polyethyleneand an ultra-high-molecular-weight ethylene/α-olefin copolymer at acertain weight ratio according to the present invention has highstrength and high elastic modulus of the former polymer and high creepresistance of the latter polymer synergistically and moreover, theinitial elongation is drastically reduced in this drawn fiber. Thesecharacteristics are quite surprising.

FIG. 1 illustrates the relation between the time elapsing afterapplication of the load and the creep elongation, which is observed withrespect to various highly drawn polyolefin fiber when a loadcorresponding to 30% of the breaking load at room temperature is appliedat a sample length of 1 cm and an ambient temperature of 70° C. In FIG.1, sample (4) is an ultra-high-molecular-weight polyethylene fiber,sample (5) is an ultra-high-molecular-weight ethylene/butene-1 copolymerfiber, and samples (1), (2) and (3) are fibers of compsitions comprisingthe above-mentioned ultra-high-molecular-weight polyethylene andultra-high-molecular-weight ethylene/butene-1 copolymer at weight ratiosof 10/20, 15/15 and 20/10, respectively. In short, the creepcharacteristics of these fibers are shown in FIG. 1. Incidentally, therespective samples are described in detail in the examples givenhereinafter.

From the results shown in FIG. 1, it is seen that in the after of thecompsition of the present invention, the initial elongation (theelongation after 60 seconds from the point of the initiation ofapplication of the load) is controlled to a much lower level even underan accelerated condition of 70° C. than in the fibers composed solely ofthe respective components.

FIGS. 2, 3, 4, 5 and 6 are temperature-melting thermal curves measuredby a differential scanning calorimeter with respect to fibers(multifilaments) of samples (1) through (5) used for the measurement ofFIG. 1 in the state where the sample is wound on an aluminum sheethaving a thickness of 0.2 mm and the end is restrained. The crystalmelting temperatures (Tm) of samples (1) through (3) according to thepresent invention, as determined as the main melting endothermic peak asthe second temperature elevation are 135.0° C., 135.6° C. and 136.2° C.,respectively. Accordingly, it is seen that the fiber of the presentinvention has, in the restrained state, a crystal melting peak only inthe region of temperatures substantially higher by at least 15° C. thanTm and this peak appears as at least two peaks close to each other. Thiscrystal melting characteristics has a close relation to drasticreduction of the initial elongation.

The fact that in the polyolefin fiber of the present invention, theinitial elongation is controlled to a very small value by blending thetwo components was accidentally found as a phenomenon, and the reason isstill unknown. However, it is presumed that the reason will probably beas follows, though the reason described below is not binding one. Ingeneral, a drawn fiber has a structure in which the polymer chain passesthrough a crystalline zone and an amorphous zone alternately and thecrystalline zone is oriented in the drawing direction, and it isconsidered that it is the amorphous zone that has influences on theinitial elongation of the fiber. In the polyolefin fiber of the presentinvention, since the fiber comprises ultra-high-molecular-weightpolyethylene and an ultra-high-molecular-weight ethylene/α-olefincopolymer, a crystal structure different from the crystal ofpolyethylene is introduced into the portion to be inherently formed intoan amporphous zone or the length of the amorphous zone is shortened. Itis considered that for this reason, the initial elongation can bereduced. As pointed out hereinbefore, the results of the differentialthermal analysis of the polyolefin fiber of the present inventionindicate formation of two phases of crystals differing in the meltingpeak.

From the viewpoint of the mechanical properties of the fiber, it isimportant that the polyolefin composition constituting the fiber of thepresent invention, as a whole, should have an intrinsic viscosity of atleast 5 dl/g, especially 7 to 30 dl/g. Since the molecule ends do notparticipate in the strength of the fiber and the number of the moleculeends is a reciprocal number of the molecular weight (viscosity), it isseen that a higher intrinsic viscosity [η] gives a higher strength.

In the present invention, it is important that the polyolefin compsitionshould comprise the ultra-high-molecular-weight ethylene/α-olefincopolymer in such an amount that the number of branched chains per 1000carbon atoms in the composition is 0.2 to 5.0 on the average, especially0.5 to 0.3 on the average. If the number of branched chains is too smalland below the above-mentioned range, it is difficult to form an internalstructure of the fiber effective for reducing the initial elongation andimproving the creep resistance. In contrast, if the number of branchedchains is too large and exceeds the above-mentioned range, thecrystallinity is drastically degraded and it is difficult to obtain highelastic modulus and strength. In the present invention, determination ofbranched chains of the composition is carried out by using an infraredspectrophotoscope (supplied by Nippon Bunko Kogyo). More specifically,the absorbance at 1378 cm⁻¹ based on the deformation vibration of themethyl group at the end of the branch of the α-olefin introduced in theethylene chain is measured and the number of branched methyl groups per1000 carbon atoms can be easily obtained from the measured value withreference to a calibration curve prepared in advance by using a modelcompound in a ¹³ C magnetic resonance apparatus.

The present invention will now be described in detail.

Starting Materials

The ultra-high-molecular-weight polyethylene used in the presentinvention is known, and any of known polymers can be optionally used. Inorder to obtain a fiber having high strength and high elastic modulus,it is preferred that the intrinsic viscosity of theultra-high-molecular-weight polyethylene be at least 5 dl/g, especially7 to 30 dl/g.

From the same viewpoint, the ultra-high-molecular-weightethylene/α-olefin copolymer as the other component should also have anintrinsic viscosity [η] of at least 5 dl/g, especially 7 to 30 dl/g.What must be taken into consideration here is that if the difference ofthe molecular weight between the ultra-high-molecular-weightpolyethylene and the ultra-high-molecular-weight ethylene/α-olefincopolymer is too large, the creep resistance of the final fiber tends todecrease. Accordingly, it is preferred that the difference of theintrinsic viscosity between both the resins be smaller than 5 dl/g,especially smaller than 3 dl/g.

As the olefin having at least 3 carbon atoms, there can be used at leastone member selected from mono-olefins such as propylene, butene-1,pentene-1, 4methlpentene-1, hexene-1, heptene-1 and octene-1.Furthermore, hydrocarbons having at least two unsaturated bonds in themolecule, preferably at least two double bonds, can be used. Forexample, there can be mentioned conjugated diene type hydrocarboncompounds such as 1,3-butadiene, 2-methyl-2,4-pentadiene,2,3-dimethyl-1,3butadiene, 2,4-hexadiene, 3-methyl-2,4-hexadiene,1,3-pentadiene and 2-methyl-1,3-butadiene, non-conjugated diene typehydrocarbon compounds such as 1,4-pentadiene, 1,5-hexadiene,1,6-heptadiene, 1,7-octadiene, 2,5-dimethyl-1,5-hexadiene, 4-methyl-1,4-hexadiene, 5-methyl-1,4-hexadiene, 4-ethyl-1,4-hexadiene,4,5-dimethyl-1,4-hexadiene,4-methyl-1,4-heptadiene,4-ethyl-1,4-heptadiene, 5-methyl-1,4-heptadiene,4-ethyl-1,4-hexadiene, 5-methyl-1,4-octadiene and4-n-propyl-1,4-decadiene, conjugated polyolefin type hydrocarboncompounds such as 1,3,5-hexatriene, 1,3,5,7-octatetraene and2-vinyl-1,3-butadiene, non-conjugated polyolefin type hydrocarboncompounds such as squalene, and divinylbenzene and vinylnorbornene.

The ultra-high-molecular-weight ethylee/α-olefin copolymer used in thepresent invention is obtained by slurry-polymerizing ethylene and anα-olefin having at least 3 carbon atoms as the comonomer in an organicsolvent by using a Ziegler type catalyst.

In this case, the amount used of the olefin comonomer should be suchthat the number of side chains (branched chains) per 1000 carbon atomsin the final composition is 0.2 to 5, especially 0.5 to 3 .

The ethylene/α-olefin copolymer most effective for attaining the objectof the present invention is an ethylene/butene-1 copolymer, and anethylene/4-methylpentene-1 copolymer, an ethylene/hexene-1 copolymer,and ethylene/octene-1 copolymer, an ethylene/propylene copolymer, andethylene/propylene/4-methylpentene-1 copolymer and anethylene/1,5-hexadiene copolymer are advantageously used. Theseultra-high-molecular-weight ethylene/α-olefin copolymers can be usedsingly or in the form of mixtures of two or more of them.

Preparation Process

In the present invention, the ultra-high-molecular-weight polyethlene(A) is combined with the ultra-high-molecular-weight ethylene/α-olefincopolymer (B) at a weight ratio (A)/(B) of from 10/90 to 90/10,especially from 20/80 to 80/20, so that the content of the α-olefinhaving at least 3 carbon atoms is such that the number of branchedchains per 1000 carbon atoms is within the above-mentioned range.

In order to make melt-shaping of the ultra-high-molecular-weight olefinresin possible, a diluent is incorporated into the composition of thepresent invention. Solvents for the ultra-high-molecular-weight olefinresin composition and various waxy substances having a compatibilitywith the ultra-high-molecular-weight olefin resin composition are usedas the diluent.

A solvent having a boiling point higher, especially by at least 20° C.,than the melting point of the above-mentioned copolymer is preferablyused.

As specific examples of the solvent, there can be mentioned aliphatichydrocarbon solvents such as n-nonane, n-decane, n-undecane, n-dodecane,n-tetradecane, n-octadecane, liquid paraffin and kerosene, aromatichydrocarbon solvents such as xylene, naphthalene tetralin, butylbenzene,p-cymene, cyclohexylbenzene, diethylbenzene, pentylbenzene,dodecylbenzene, bicyclohexyl, decalin, methylnaphathalene andethynaphthalene, hydrogenated derivatives thereof, halogenatedhydrocarbon solvents such as 1,1,2,2-tetrachloroethane,pentachloroethane, hexachloroethane, 1,2,3-trichloropropane,dichlorobenzene, 1,2,4-trichlorobenzene and bromobenzene, and mineraloils such as paraffin type process oil, naphthene type process oil andaromatic process oil.

Aliphatic hydrocarbon compounds and derivatives thereof are used as thewax.

The aliphatic hydrocarbon compound is a so-called paraffin wax composedmainly of a staurated aliphatic hydrocarbon compound and having amolecular weight lower than 2000, preferably lower than 1000, especiallypreferably lower than 800. As specific examples of the aliphatichydrocarbon compound, there can be mentioned n-alkanes having at least22 carbon atoms, such as docosane, tricosane, tetracosane andtriacontane, mixtures containing an n-alkane as mentioned above as themain component and a lower n-alkane, so called paraffing waxes separatedand purified from petroleum, low-pressure and medium-pressure polyolefinwaxes which are low-molecular-weight polymers obtained by polymerizingethylene or copolymerizing ethylene with other α-olefin, high-pressurepolyethylene waxes, ethylene copolymer waxes, waxes obtained by reducingthe molecular weight of polyethylene such as medium-pressure,low-pressure or high-pressure polyethylene by thermal degradation or thelike, and oxidized waxes and maleic acid-modified waxes obtained byoxidizing the foregoing waxes or modifying the foregoing waxes withmaleic acid.

As the hydrocarbon derivative, there can be mentioned fatty acids,aliphatic alcohols, fatty acid amides, fatty acid esters, aliphaticmercaptans, aliphatic aldehydes and aliphatic ketones having at least 8carbon atoms, preferably 12 to 50 carbon atoms, or a molecular weight of130 to 2000, preferably ®to 800, which are compounds having at theterminal of an aliphatic hydrocarbon group (such as an alkyl or alkenylgroup) or in the interior thereof, at least one, preferably one or two,especially preferably one, of functional groups such as a carboxylgroup, a hydroxyl group, a carbamoyl group, and ester group, a mercaptogroup and a carbonyl group.

As specific examples, there can be mentioned fatty acids such as capricacid, lauric acid, myristic acid, palmitic acid, stearic acid and oleicacid, aliphatic alchols such as lauryl alchol, myristyl alcohol, cetylalcohol and stearyl alcohol, fatty acid amides such as caprylamide,laurylamide, palmitylamide and stearylamide, and fatty acid esters suchas stearyl acetate.

The ratio between the ultra-high-molecular-weight olefin resincomposition and the diluent differs according to the kinds of them, butit is generally preferred that the above-mentioned ratio is in therange, of from 3/97 to 80/20, especially from 15/85 to 60/40. If theamount of the dilluent is too small and below the above-mentioned range,the melt viscosity becomes too high, and melt kneading or melt shapingbecomes difficult and such troubles as surface roughening of the shapedbody and breaking at the drawing step are often caused. If the amount ofthe diluent is too large and exceeds the above-mentioned range, meltkneading is difficult and the drawability of the shape body is poor.

It is preferred that melt kneading be carried out at a temperature of150° to 300°C., especially 170° to 270° C. If the temperature is too lowand below the above-mentioned range, the melt viscosity is too high andmelt shaping becomes difficult. If the temperature is too high andexceeds the above-mentioned range, the molecular weight of theultra-high-molecular-weight olefin compositon is reduced by thermaldegradation and a shaped body having a high elastic modulus and a highstrength can hardly be obtained. Mixing can be accomplished by dryblending using a Henschel mixer or a V-type blender or by melt mixingusing a single-screw or multiple-screws extruder.

Melt shaping is generally accomplished according to the melt extrusionshaping method. For example, filaments to be drawn can be obtained bymelt extrusion through a spinneret. In this case, a melt extruded from aspinneret may be drafted, that is , elongated in the molten state. Thedraft ratio can be defined by the following formula

    draft ratio=V/Vo                                           (1)

wherein Vo stands for the extrusion speed of the molten resin in a dieorifice and V for the winding speed of the cooled and solidified undrawnbody.

The draft ratio is changed according to the temperature of the mixture,the molecular weight of the ultra-high-molecular-weight olefin resincomposition and the like, but the draft ratio can be ordinarily adjustedto at least 3, preferably at least 6.

The so-obtained undrawn shaped body of the ultra-high-molecular-weightolefin resin composition is subjected to a drawing operation. The degreeof drawing is, of course, such that a molecular orientation iseffectively given in at least one axial direction of the drawn fiber ofthe ultra-high-molecular-weight olefin composition.

It is generally preferred that drawing of the shaped bofy of theultra-high-molecular-weight olefin resin composition be carried out at40° to 160° C., eskpecially 80° to 145° C. As the heating medium forheating and maintaining the undrawn shaped body at the above-mentionedtemperature, there can be used any of air, steam and liquid media. Ifthe drawing operation is carried out by using, as the heating medium, amedium capable of removing the above-mentioned diluent by extraction andhaving a boiling point higher than that of the composition constitutingthe shaped body, such as decalin, decane or kerosene, removal of thediluent becomes possible, and drawing uneveness can be eliminated at thedrawing step and a high draw ratio can be adopted. Accordingly, use ofthe above-mentioned medium is preferred.

The means for removing the excessive diluent from theultra-high-molecular-weight olefin resin composition is not limited tothe above-mentioned method. For example, the excessive diluent can beeffectively removed according to a method in which the undrawn shapedbody is treated with a solvent such as hexane, heptane, hot ethanol,chloroform or benzene and is then drawn, or a method in which the drawnshaped body is treated with a solvent such as hexane, heptane, hotethanol, chloroform or benzene, whereby a drawn shaped body having ahigh elastic modulus and a high strength can be obtained.

The drawing operation can be performed in a single stage or two or morestages. The draw ratio depends on the desired molecular orientation andthe effect of improving the melting temperature characteristic by themolecular orientation. In general, however, satisfactory results can beobtained if the drawing operation is carried out so that the draw ratiois 5 to 80, especially 10 to 50.

In general, multi-staged drawing conducted in at least two stags isadvantageous. Namely, it is preferred that at the first stage, thedrawing operation be carried out at a relatively low temperature of 80to 120° C. while extracting the diluent contained in the extruded shapedbody and at the second and subsequent stages, drawing of the shaped bodybe carried out at a temperature of 120° to 160° C., which is higher thanthe drawing temperature adopted at the first stage.

The uniaxial drawing operation for a filament can be accomplished bystretch-drawing between rollers differing in the peripheral speed.

The so-obtained molecularly oriented shaped body can be heat-treatedunder restrained conditions, if desired. This heat treatment isgenerally carried out at a temperature of 140° to 180° C., especially150° to 175° C., for 1 to 20 minutes, especially 3 to 10 minutes. Bythis heat treatment, crystallization of the oriented crystal zone isfurther advanced, the crystal melting temperature is shifted to the hightemperature side, and the strength and elastic modulus and the creepresistance at high temperatures are improved.

Drawn Fiber

As pointed out hereinbefore, the drawn fiber of the present invention ischaracterized in that the fiber has at least two crystal meltingendothermic peaks, close to each other, in the region of temperatureshigher by at least 15° C. than the crystal melting temperature (Tm) ofpolyethylene determined as the main melting endothermic peak at thesecond temperature elevation, when measured in the restrained state by adifferential scanning calorimeter. By dint of this specific crystalstructure, the fiber of the present invention can have such surprisingcharacteristics that when a load corresponding to 30% of the breakingload at room temperature is applied at a sample length of 1 cm and anambient temperature of 70° C., the initial elongation after 60 secondsfrom the point of the initiation of application of the load is lowerthan 5%, especially lower than 4%, and the average creep speed duringthe period of from the point of 90 seconds from the initiation ofapplication of the load to the point of 180 seconds from the initiationof application of the load is lower than 1×10⁻⁴ sec⁻¹, especially lowerthan 7×10⁻⁵ sec⁻¹.

The inherent crystal melting temperature (Tm) of theultra-high-molecular-weight olefin resin composition can be determinedaccording to a method in which the shaped body is completely molten onceand then cooled to moderate the molecular orientation in the shaped bodyand the temperature is elevated again, that is, by the second run in aso-called differential scanning calorimeter.

In the present invention, the melting point and crystal melting peak aredetermined according to the following methods.

The melting point is measured by using a differential scanningcalorimeter (Model DSC II supplied by Perkin-Elmar) in the followingmanner. About 3 mg of a sample was wound on an aluminum plate having asize of 4 mm×4 mm×0.2 mm (thickness) to restrain the sample in theorientation direction. Then, the sample wound on the aluminum plate issealed in an aluminum pan to form a measurement sample. The samealuminum plate is sealed in an empty aluminum pan to be placed in areference holder, whereby a thermal balance is maintained. At first, thesample is maintained at 30° C. for about 1 minute, and then, thetemperature is elevated to 250° C. at a temperature-elevating rate of10° C./min to complete the measurement of the melting point at the firsttemperature elevation. Subsequently, the sample is maintained at 250° C.for 10 minutes, and the temperature is dropped at a temperature-droppingrate of 20° C./min and the sample is maintained at 30° C. for 10minutes. Then, the second temperature elevation is carried out byelevating the temperature to 250° C. at a temperature-elevating rate of10° C./min to complete the measurement of the melting point at thesecond temperature elevation (second run). The maximum value of themelting peak is designated as the melting point. In the case where thepeak appears as a shoulder, tangential lines are drawn on the bendingpoint just on the low temperature side of the shoulder and on thebending point just on the high temperature side of the shoulder, and thepoint of intersection is designated as the melting point.

In the differential thermal curve of the present invention, theendothermic peak (T_(H)) appearing on the high temperature side isconsidered to be an inherent peak of crystalline polyethylene segmentsand the endothermic peak (T_(L)) appearing on the low temperature sideis considered to be an inherent peak of the crystallizedethylene/α-olefin copolymer segments. The temperatures at which T_(H)and T_(L) appear differ according to the mixing ratio and theorientation degree, but these temperatures are generally as follows.

    ______________________________________                                                 General Range                                                                           Preferred Range                                            ______________________________________                                        T.sub.H    150 to 157° C.                                                                     151 to 156° C.                                  T.sub.L    149 to 155° C.                                                                     150 to 154° C.                                  T.sub.H -T.sub.L                                                                         2.5 to 0.5° C.                                                                     2.0 to 1.0° C.                                  ______________________________________                                    

Some fibers obtained by spinning an ethylene/α-olefin copolymer anddrawing the fiber at a high draw ratio show two endothermic peaks, butin these fibers, the high-temperature side peak (T_(H)) is lower than incase of the fiber of the present invention, and the difference (T_(H)-T_(L)) between the two peak temperatures is larger than in the fiber ofthe present invention.

The ratio of the height (I_(H)) of the peak on the high temperature sideto the height (I_(L)) of the peak on the low temperature side in thedifferential thermal curve should naturally differ according to theblend ratio of both the resins, but it is generally preferred that theI_(H) /I_(L) ratio be in the range of from 1.5 to 0.5, especially from1.4 to 0.6.

The degree of molecular orientation in the shaped body can be known bythe X-ray diffractometry, the birefringence method, the fluorescencepolarization method or the like. The drawn filament of theultra-high-molecular-weight olefin resin composition according to thepresent invention is characterized in that the orientation degree by thehalf width in the X-ray diffractometry, described in detail, forexample, in Yukichi Go and Kiichiro Kubo, Kogyo Kagaku Zasshi, 39, 922(1939), that is, the orientation degree (F) defined by the followingformula: ##EQU1## wherein H° stands for the half width (°) of theintensity distribution curve along the Debye ring of the strongestparatroope plane on the equator line,

is at least 0.90, preferably at least 0.95.

The drawn filament of the ultra-high-molecular-weight olefin resincomposition has such a heat resistance characteristic that the strengthretention ratio after the heat history at 170° C. for 5 minutes is 90%,especially at least 95%, and the elastic modulus retention ratio is atleast 90%, especially at least 95%. This excellent heat resistance isnot attained in any of conventional drawn polyethylene filaments.

The drawn filament of the ultra-high-molecular-weight olefin resincomposition of the present invention is excellent in the mechanicalcharacteristics. Namely, the drawn fiber of theultra-high-molecular-weight olefin resin composition of the presentinvention has an elastic modulus of at least 30 GPa, especially at least50 GPa, and a tensile strength of at least 1.5 GPa, especially at least2.0 GPa.

The drawn fiber of the present invention can be used in the form of amonofilament, multifilament or staple for cords, ropes, woven fabricsand non-woven fabrics or as a reinforcer for various rubbers, resins,cements and the like.

The composition comprising ultra-high-molecular-weight polyethylene andan ultra-high-molecular-weight ethylene/α-olefin copolymer according tothe present invention has good spinnability and drawability and can beshaped into a highly drawn filament, and the obtained fiber is excellentin the combination of high strength, high elastic modulus and high creepresistance, and furthermore, the initial elongation can be controlled toa very low level.

Accordingly, if the fiber of the present invention is used as a stresscarrier of a fiber-reinforced composite body or other composite body,high strength and high elastic modulus of the fiber can be effectivelyutilized.

The present invention will now be described in detail with reference tothe following examples that by no means limit the scope of theinvention.

EXAMPLE 1

A mixture comprising a powder of an ultra-high-molecular-weight ethylenehomopolymer (intrinsic viscosity [η]=8.73 dl/g), a powder of anultra-high-molecular-weight ethylene/butene-1 copolymer (intrinsicviscosity [η]=9.26 dl/g, butene-1 content=2.4 branched chains per 1000carbon atoms) and a powder of a paraffin wax (melting point=69° C.,molecular weight=490) was melt-spun under conditions described below.The mixing ratio of the starting materials is shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________          Ultra-High-  Ultra-High-Molecular-                                            Molecular-Weight                                                                           Weight Ethylene/                                                                         Paraffin                                              Ethylene Homopolymer                                                                       Butene-1 Copolymer                                                                       Wax (parts                                      Sample No.                                                                          (parts by weight)                                                                          (parts by weight)                                                                        by weight)                                      __________________________________________________________________________    1     10           20         70                                              2     15           15         70                                              3     20           10         70                                              __________________________________________________________________________

Prior to spinning, 0.1 part by weight of3,5-dimethyl-tert-butyl-4-hydroxytoluene was added in an amount of 0.1part by weight as a process stabilizer homogeneously into the mixture.

Then, the mixture was melt-kneaded at a set temperature of 190° C. byusing a screw type extruder (screw diameter=25 mm, L/D=25; supplied byThermoplastic Kogyo), and subsequently, the melt was melt-spun from aspinning die having an orific diameter of 2 mm, which was attached tothe extruder. The spun fiber was taken up under drafting conditions inan air gap having a length of 180 mm, and was then cooled and solidifiedin air to obtain an undrawn fiber shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Sample No.                                                                            Fineness (denier)                                                                            Draft Ratio                                                                             Spinnability                                 ______________________________________                                        1       593            47        good                                         2       643            43        good                                         3       643            44        good                                         ______________________________________                                    

The undrawn fiber was drawn under conditions described below to obtainan oriented fiber. Namely, three-staged drawing was carried out by usingfour sets of godet rolls. At this drawing operation, the heating mediumin first and second drawing tanks was n-decane and the temperatures inthe first and second tanks were 110° C. and 120° C., respectively. Theheating medium of a third drawing tank was triethylene glycol, and thetemperature in the third tank was 145° C. The effective length of eachtank was 50 cm. At the drawing operation, the rotation speed of thefirst godet roll was set at 0.5 m/min, and a fiber having a desired drawratio was obtained by adjusting the rotation speed of the fourth godetroll. The rotation speeds of the second and third godet rolls wereappropriately arranged within such a range that drawing could be stablyperformed. The majority of the paraffin wax mixed at the initial stagewas extracted out in the n-decane tanks.

Incidentally, the draw ratio was calculated from the rotation speedratio between the first and fourth godet rolls.

Measurement of Tensile Characteristics

The elastic modulus and tensile strength were measured at roomtemperature (23° C.) by using a tensile tester (Model DCS-50M suppliedby Shimazu Seisakusho). The sample length between clamps was 100 mm, andthe pulling speed was 100 mm/min. The elastic modulus was calculatedfrom the initial elastic modulus by using the gradient of the tangent.The cross-sectional area of the fiber necessary for the calculation wasdetermined based on the presumption that the density of the fiber was0.960 g/cc.

Measurement of Creep Resistance Characteristic and Initial Elongation

The creep test was carried out at a sample length of 1 cm and an ambienttemperature of 70° C. by using a thermal stress distortion measuringapparatus (Model TMA/SS10 supplied by Seiko Denshi Kogyo) under such anaccelerated load condition that a load corresponding to 30% of thebreaking load at room temperature was applied. In order toquantitatively evaluate the creep quantity and initial elongation, theelongation EL-60 (%) after 60 seconds from the point of the initiationof application of the load, corresponding to the initial elongationbefore entrance into the stationary creep state, and the average creepspeed ε (sec⁻¹) during the period of from the point of 90 seconds fromthe initiation of application of the load to the point of 180 secondsfrom the initiation of application of the load, in which the stationarycreep state had already been brought about, were determined.

The tensile characteristics of the sample and the initial elongation andcreep characteristics of the sample are shown in Tables 3 and 4,respectively.

                                      TABLE 3                                     __________________________________________________________________________                               Elastic                                                                  Strength                                                                           Modulus                                                                            Elongation                                    Sample No.                                                                          Draw Ratio                                                                           Fineness(denier)                                                                       (GPa)                                                                              (GPa)                                                                              (%)                                           __________________________________________________________________________    1     15     10.4     2.4  62.2 4.6                                           2     18     10.1     2.7  69.4 4.8                                           3     21     10.1     2.8  72.1 4.8                                           __________________________________________________________________________

                  TABLE 4                                                         ______________________________________                                        Sample No.    EL-60 (%) ε (sec.sup.-1)                                ______________________________________                                        1             2.64      1.49 × 10.sup.-5                                2             3.12      2.22 × 10.sup.-5                                3             3.46      5.56 × 10.sup.-5                                ______________________________________                                    

As is apparent from the comparison of the results obtained in thepresent example with the results obtained in the comparative examplegiven hereinafter, in the fiber of the present invention, the initialelongation is improved over that of the fiber formed from theultra-high-molecular-weight polyethylene or ultra-high-molecular-weightethylene/butene-1 copolymer alone, and the creep resistance of sample 1is much improved over that of the fiber composed solely of theultra-high-molecular-weight ethylene/butene-1 copolymer. The inherentmain crystal melting temperatures (Tm) of the compositions of samples 1through 3 were 135.0° C., 135.6° C. and 136.2° C., respectively.Furthermore, the I_(H) /I_(L) ratios of samples 1 through 3 were 1.10,1.28 and 0.73, respectively.

COMPARATIVE EXAMPLE 1

The ultra-high-molecular-weight ethylene homopolymer andultra-high-molecular-weight ethylene/butene-1 copolymer described inExample 1 were independently melt-spun in the same manner as describedin Example 1. The mixing ratios between the polymer and wax are shown inTable 5.

                                      TABLE 5                                     __________________________________________________________________________          Ultra-High-  Ultra-High-Molecular-                                            Molecular-Weight                                                                           Weight Ethylene/                                                                         Paraffin                                              Ethylene Homopolymer                                                                       Butene-1 Copolymer                                                                       Wax (parts                                      Sample No.                                                                          (parts by weight)                                                                          (parts by weight)                                                                        by weight)                                      __________________________________________________________________________    4     30           --         70                                              5     --           30         70                                              __________________________________________________________________________

The undrawn fibers obtained by spinning the mixtures shown in Table 5are shown in Table 6.

                  TABLE 6                                                         ______________________________________                                        Sample No.                                                                            Fineness (denier)                                                                            Draft Ratio                                                                             Spinnability                                 ______________________________________                                        4       650            40        good                                         5       892            35        good                                         ______________________________________                                    

The tensile characteristics of the fibers obtained by drawing theundrawn fibers shown in Table 6 are shown in Table 7, and the initialelongation and creep characteristics of these drawn fibers are shown inTable 8.

                                      TABLE 7                                     __________________________________________________________________________                               Elastic                                                                  Strength                                                                           Modulus                                                                            Elongation                                    Sample No.                                                                          Draw Ratio                                                                           Fineness(denier)                                                                       (GPa)                                                                              (GPa)                                                                              (%)                                           __________________________________________________________________________    4     25      7.8     3.1  77.6 5.4                                           5     12     22.3     2.3  42.3 6.3                                           __________________________________________________________________________

                  TABLE 8                                                         ______________________________________                                        Sample No.    EL-60 (%) ε (sec.sup.-1)                                ______________________________________                                        4             5.40      4.51 × 10.sup.-5                                5             4.44      2.89 × 10.sup.-5                                ______________________________________                                    

The inherent main crystal melting temperatures (Tm) of the compositionsof samples 4 and 5 are 137.5° C. and 134.8° C., respectively, and theI_(H) /I_(L) ratio of the sample 5 was 1.45.

We claim:
 1. A drawn, high strength polyolefin fiber comprising a drawncomposition which is blend of (A) ultra-high-molecular-weightpolyethylene having an intrinsic viscosity (η) of at least 5 dl/g with(B) ultra-high-molecular-weight ethylene/α-olefin copolymer having anintrinsic viscosity (η) of at least 5 dl/g and containing 0.5 to 10α-olefin groups of at least 3 carbon atoms per 1000 carbon atoms onaverage, at an (A):(B) weight ratio of 10:90 to 90:10, said compositionbefore drawing having an intrinsic viscosity (η) of at least 5 dl/g as awhole, and containing 0.2 to 5.0 α-olefin groups of at least 3 carbonatoms as side chains per 1000 carbon atoms on the average, and saiddrawn fiber having the following properties: (i) when measured underrestraint conditions using a differential scanning calorimeter, has atleast two close crystal melting peaks at temperatures higher by at least15° C. than the inherent crystal melting temperature (Tm) of thepolyethylene determined as the main peak at the time of the secondtemperature elevation, (ii) an initial elongation of less than 5% whenmeasured 60 second from the time of initiation of application of a load,corresponding to 30% of the breaking load applied at room temperature toa test sample 1 cm long, at ambient temperature of 70° C., (iii) anaverage creep rate of at least 1×10⁻⁴ sec⁻¹ when measured over theperiod of from 90 to 180 seconds after the time of initiating theapplication of said load, (iv) a strength retention ratio of at least90% when measured after a heat history at 170° C. for 5 minutes, (v) anelastic modulus of at least 30 GPa at room temperature, and (vi) atensile strength of at least 1.5 GPa.
 2. A polyolefin fiber as set forthin claim 1, wherein of said at least two crystal melting endothermicpeaks, the endothermic peak (T_(H)) on the high temperature side and theendothermic peak (T_(L)) on the low temperature side appear attemperatures satisfying the requirements of T_(H) =150° to 157° C.,T_(L) =149° to 155° C. and T_(H) -T_(L) =2.5° to 0.5° C., and the ratio(I_(H) /I_(L)) of the height (I_(H)) of the peak on the high temperatureside to the height (I_(L)) of the peak on the low temperature side is inthe range of from 1.5 to 0.5.
 3. A polyolefin fiber as set forth inclaim 2, wherein T_(H) and T_(L) appear at temperatures satisfying therequirements of T_(H) =151° to 156° C., T_(L) =150° to 154° C. and T_(H)-T_(L) =2.0° to 1.0° C., and the ratio I_(H) /I_(L) is in the range offrom 1.4 to 0.6.
 4. The drawn, high strength fiber of claim 1 whereinthe composition of (A) and (B) has an intrinsic viscosity of from 7 to30 dl/g and from 0.5 to 3.0, on average, of α-olefin groups of at least3 carbon atoms as side chains per 1000 carbon atoms, and wherein thedifference in the intrinsic viscosity between polyethylene (A) andcopolymer (B) is less than 3 dl/g.
 5. The drawn, high strengthpolyolefin fiber of claim 1 which has been drawn to a draw ratio of atleast 10.